An internal combustion engine, particularly, an automotive engine will produce lots of heat when it's running. An engine cooling system is used to help discharging the heat, to prevent the heat damaging components of the engine. An engine cooling system usually comprises a coolant tank, cooling pipes and a fan unit. The heat is absorbed by coolant passing through cooling channels in the engine. After absorbing heat, the coolant circles to the coolant tank via the cooling pipes. The cooling pipes are cooled by a fan unit. The fan unit mainly comprises a motor and fan blades driven by the motor. The motor comprises a permanent magnet stator and a wound rotor.
A motor's windings are usually wound by copper wire, since copper wire has a good electrical conductivity, good strength and an acceptable chemical stability. The windings formed by winding copper wire are hereinafter called copper windings. Copper wire has a high hardness and sometimes, laminations such as a rotor core will be deformed by the copper windings during assembly. FIG. 6 shows a core of a rotor which had been wound with copper windings. The windings have been removed to show the distortion which can occur to the teeth of the rotor by copper wire during the winding process, the deformation is especially noticeable when the teeth are long. In particular, the force applied to the rotor teeth by the copper wire causes the outer laminations to bend, distorting the effective position of the center of the tooth which affects the commutation angle and thus the life and efficiency of the motor.
Furthermore, copper costs more than aluminum. In the past during hard economic times, it has been known to make windings from aluminum wire. The windings formed from aluminum wire are hereinafter called aluminum windings.
However, the use of aluminum windings will encounter many difficulties. It's difficult to solder aluminum wire to commutator segments which are usually made of copper. It is also difficult to attach aluminum windings to commutator hooks by the common welding or fusing techniques due to the soft nature of aluminum wire. Therefore, aluminum windings were used in brushless DC motor as a part of motor stator, instead of motor rotor. This will limit the use of aluminum windings.
Secondly, aluminum has a strong chemical activity and is vulnerable to corrosion and oxidation. Lead wires of aluminum wire are easily to oxidize, resulting in a higher resistivity. Therefore, the lead wires will become too hot when the motor is running, which may result in a motor failure or damage to parts of the motor.
In addition, aluminum has a higher resistivity than copper, and aluminum wire has a higher resistance than copper wire. Accordingly, under the same terminal voltage, the current in aluminum windings will be less than the current in copper windings. Therefore, performance of a motor using aluminum windings is affected. In other words, to make the current in aluminum windings equal to the current in copper windings, it is necessary to increase the diameter of aluminum wire. It will use more aluminum materials and increase the size of the motor.
Therefore, most motors, especially the motors used in automotive engine cooling application field, are equipped with copper windings. For an automotive engine cooling system, sometimes the motor stalls, for example, when the motor is frozen by low ambient temperature or its fan blades are blocked by an obstacle. In these circumstances, the current in the windings will be too large when the motor is powered on, and it can produces a lot of heat in a very short time such as 60 seconds, which will likely damaged the motor, resulting in cooling system failure.